Negative electrode active material for secondary batteries, and secondary battery

ABSTRACT

A negative electrode active material for a secondary battery includes silicate composite particles each of which contain a silicate phase and silicon particles dispersed in the silicate phase, the silicate phase is an oxide phase containing Si, O, and alkali metals, and the alkali metals include at least Na and K.

TECHNICAL FIELD

The present invention relates primarily to an improvement of a negativeelectrode active material for a secondary battery.

BACKGROUND ART

In recent years, since having a high voltage and a high energy density,a secondary battery has been expected to be used as an electric powersource of a compact consumer use, an electric power storage device,and/or an electric car. In an environment in which an increase in energydensity of a battery has been pursued, as a negative electrode activematerial having a high theoretical capacity density, the use of amaterial containing silicone (Si) which forms an alloy with lithium hasbeen anticipated. In particular, since a material (SiO_(x)) in whichfine silicon particles are dispersed in SiO₂ can be suppressed frombeing miniaturized due to expansion and contraction of silicon, muchattention has been paid on this material.

However, since having a high irreversible capacity, SiO_(x) has aproblem in terms of a low initial charge/discharge efficiency. Inaddition, since an amount of silicon to be contained in SiO_(x) which issynthesized by a vapor phase method is restricted, a material having anx value of approximately 1 may only be obtained, and an increase incapacity is restricted.

Accordingly, the use of composite particles each of which contain alithium silicate phase containing lithium in advance in an amountcorresponding to the irreversible capacity and silicon particlesdispersed in the lithium silicate phase has been proposed (PatentDocument 1). The silicon particles contribute to a charge/dischargereaction (reversible occlusion and release of lithium). The compositeparticles each containing the lithium silicate phase and the siliconparticles are manufactured by sintering a mixture of a glassy lithiumsilicate powder and silicon particles at a high temperature and in ahigh pressure atmosphere. The amount of the silicon particles containedin the composite particle can be arbitrarily controlled by a mixing ratebetween the lithium silicate powder and the silicon particles.

CITATION LIST Patent Document

Patent Document 1: Japanese Published Unexamined Patent Application No.2015-153520

SUMMARY OF INVENTION

In the sintering step, the lithium silicate powder is melted to flow soas to fill voids formed between the silicon particles. As a result, asea-island structure in which the lithium silicate phase functions as asea portion and the silicon particles function as island portions isformed. Since a dense sea-island structure is formed, the compositeparticles are suppressed from being degraded due to expansion andcontraction of the silicon particles. That is, in order to improve cyclecharacteristics of a battery, dense composite particles are required tobe formed.

In the sintering step, a decrease in viscosity of the lithium silicateat a sintering temperature is required. In addition, the crystallizationof the lithium silicate may be advanced. When the lithium silicate ispartially crystallized, the fluidity thereof may be decreased, and thevoids formed between the silicon particles may not be sufficientlyfilled in some cases. As a result, composite particles having voids aregenerated, and the cycle characteristics of the battery become difficultto sufficiently improve.

In consideration of those described above, one aspect of the presentinvention relates to a negative electrode active material for asecondary battery, the negative electrode active material comprisingsilicate composite particles each of which contain a silicate phase andsilicon particles dispersed in the silicate phase. In the negativeelectrode active material described above, the silicate phase is anoxide phase which contains Si, O, and alkali metals, and the alkalimetals include at least Na and K.

According to the present disclosure, the crystallization of the silicatein the sintering step is suppressed, and silicate composite particleshaving a small number of voids can be obtained; hence, a secondarybattery excellent in cycle characteristics can be obtained.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a cross-sectional view schematically showing a silicatecomposite particle according to one embodiment of the presentdisclosure.

FIG. 2 is a partially notched perspective view of a secondary batteryaccording to one embodiment of the present disclosure.

DESCRIPTION OF EMBODIMENTS

A negative electrode active material for a secondary battery accordingto an embodiment of the present disclosure includes silicate compositeparticles each of which contain a silicate phase and silicon particlesdispersed in the silicate phase. In other words, the silicate compositeparticles each include a silicate phase functioning as a sea portion ofa sea-island structure and silicon particles functioning as islandportions thereof. The silicate phase is an oxide phase which containsSi, O, and alkali metals. In this embodiment, as the alkali metals, thesilicate phase contains at least Na and K.

In general, a glass containing a plurality of alkali metals has a lowion conductivity as compared to that of a glass containing a singlealkali metal. The phenomenon as described above is called a mixed alkalieffect. Silicate composite particles to be used as a negative electrodeactive material of a battery are required to have a predetermined ionconductivity. Hence, in the past, a silicate phase containing Si, O, anda single alkali metal has been believed to be advantageous.

However, in practice, when the silicate phase contains at least Na andK, various merits can be obtained. As a first merit, a silicate phasecontaining Na and K can be manufactured at a low cost compared to thatin the past. The reasons for this are that the resources of Na and K areabundant, and raw materials thereof are each commercially available at alow cost.

As a second merit, the silicate phase containing Na and K is not likelyto be crystallized, the viscosity in a molten state is low, and thefluidity is excellent. Accordingly, in a sintering step, voids formedbetween the silicon particles are likely to be filled, and densecomposite particles are likely to be generated.

As a third merit, when the content of the silicon particles contained inthe silicate composite particle is increased, and when the rate of theisland portions is increased, the flow of the silicate in a molten stateis disturbed. Hence, the use of a silicate having an excellent fluidityis further required. Since the silicate containing Na and K is excellentin fluidity, even when the rate of the island portions is high, thissilicate is likely to fill the voids formed between the siliconparticles. That is, since the silicate containing Na and K is used,dense silicate composite particles having a high capacity are likely tobe obtained.

In addition, even when the silicate phase containing Na and K is used,compared to the case in which a silicate phase containing a singlealkali metal is used, a battery reaction is not disturbed, and asufficient battery performance can be obtained.

An atomic ratio: Na/K of Na to K contained in the silicate phase may be,for example, 0.1 to 7 or may also be 0.4 to 2. When the Na/K ratio isset in the range described above, the crystallization of the silicatephase is further suppressed, and the fluidity of the silicate phase in amolten state can be further improved.

With respect to the total amount of elements other than O contained inthe silicate phase, for example, the content of Na is 7 percent by moleor more, the content of K is 7 percent by mole or more, and the totalcontent of Na and K is 70 percent by mole or less. Since the contents ofthe elements are set in the ranges described above, a more inexpensivesilicate phase excellent in fluidity in a molten state is likely to beobtained. The content of Na may also be 10 percent by mole or more, andthe content of K may also be 10 percent by mole or more. In addition,the total content of Na and K may be 60 percent by mole or less or mayalso be 50 percent by mole or less.

The alkali metals may further include Li. Since two types or at leastthree types of alkali metals including Na and K are contained, asilicate phase more excellent in fluidity in a molten state can begenerated. In addition, in order to highly efficiently suppress thecrystallization of the silicate phase, the content of Li with respect tothe total amount of the elements other than O contained in the silicatephase may be, for example, set to 30 percent by mole or less or may alsobe set to less than 20 percent by mole (such as 15 percent by mole orless). In addition, in view of reduction in cost, a lower content ofexpensive Li is more advantageous.

When the silicate phase contains two types or at least three types ofalkali metals, among the alkali metals, the total content of alkalimetals other than a first alkali metal having a highest content withrespect to the total amount of the elements other than O contained inthe silicate phase may be, for example, 7 percent by mole or more, maybe 10 percent by mole or more, or may also be 15 percent by mole ormore. In addition, the content of all the alkali metals may be 70percent by mole or less, may be 50 percent by mole or less, or may alsobe 40 percent by mole or less.

The silicate phase may further contain a group 2 element. In general,although a silicate phase has an alkali property, the group 2 elementhas a function to suppress elution of an alkali metal from the silicatephase. Hence, when a slurry containing a negative electrode activematerial is prepared, a slurry viscosity is likely to be stabilized. Atreatment (such as an acid treatment) to neutralize an alkali componentof the silicate composite particles is not so much necessary.

As the group 2 element, at least one selected from the group consistingof Be, Mg, Ca, Sr, Ba, and Ra may be used. Among those mentioned above,since a Vickers hardness of the silicate phase is improved, and thecycle characteristics are further improved, Ca is preferable.

With respect to the total amount of the elements other than O containedin the silicate phase, the content of the group 2 element is, forexample, 20 percent by mole or less and may be 15 percent by mole orless, may be 10 percent by mole or less, or may also be 5 percent bymole or less or 4 percent by mole or less.

The silicate phase may further contain an element M other than thosementioned above. The element M may be at least one selected from thegroup consisting of B, Al, Zr, Nb, Ta, La, V, Y, Ti, P, Bi, Zn, Sn, Pb,Sb, Co, Er, F, and W. Among those mentioned above, B has a low meltingpoint and is advantageous to improve the fluidity of the silicate in amolten state. In addition, Al, Zr, Nb, Ta, and La each may improve theVickers hardness while the ion conductivity of the silicate phase ismaintained.

In consideration of the fluidity in a molten state, the silicate phaseis preferably in an amorphous state in the sintering step. In an X-raydiffraction pattern of the silicate composite particles after thesintering step, a ratio of an integrated value of a diffraction peak ofeach of all the other elements to an integrated value of a diffractionpeak which belongs to the (111) plane of single Si is 0.5 or less andmay be 0.3 or less, may be 0.1 or less, or may also be 0 (zero). Inaddition, the diffraction peak which belongs to the (111) plane ofsingle Si is observed at approximately 2θ=28°.

According to the embodiment described above, a void rate of the silicatecomposite particle is, for example, less than 5 percent by volume andmay be set to 3 percent by volume or less or may also be 1 percent byvolume or less.

When the void rate is measured, after the silicate composite particlesare immersed in a liquid paraffin (such as a paraffin having a specificgravity of approximately 0.85), vacuum decompression is performed, andair in the voids are replaced with the liquid paraffin. Subsequently,from the different in specific gravity before and after the immersion,the void rate is obtained. In particular, an increase in specificgravity depends on the mass of the liquid paraffin which enters thevoids. From the mass of the liquid paraffin which enters the voids, thevolume of the voids can be calculated. As another measurement method ofthe void rate, after a cross-section of the silicate composite particleis obtained by a cross-section polisher (CP), an area rate (void rate)of the voids occupied in the cross-section of the composite particle maybe obtained by a cross-sectional image analysis using a SEM. Forexample, after 10 silicate composite particles each having a maximumdiameter of 5 μm or more are randomly selected, the void rates thereofare obtained, and an average value of the 10 void rates may becalculated.

Next, the contents of B, Na, K, and Al contained in the silicate phaseare quantitatively analyzed in accordance with JIS 83105 (1995)(analysis method of borosilicate glass), and the content of Ca isquantitatively analyzed in accordance with JIS 83101 (1995) (analysismethod of soda-lime glass).

The other elements to be contained in the silicate phase may bequantified by the following method. First, a sample of the silicatephase or the silicate composite particle containing the same iscompletely melted in a heated acid solution (mixed acid of hydrofluoricacid, nitric acid, and sulfuric acid), and carbon which is a residue ofthe solution is removed by filtration. Next, a filtrate thus obtained isanalyzed by an inductively-coupled plasma atomic emission spectrometry(ICP-AES), and a spectral intensity of each element is measured.Subsequently, a calibration curve is formed using a commerciallyavailable standard solution of the element, and the content of eachelement contained in the silicate phase is calculated.

In the silicate composite particle, although the silicate phase and thesilicon particles are present, by using Si-NMR, the two componentsdescribed above can be separately quantitatively measured. The contentof Si obtained by ICP-AES as described above indicates the total of theSi amount forming the silicon particles and the Si amount in thesilicate phase. On the other hand, the Si amount forming the siliconparticles can be separately quantitatively measured using Si-NMR. Hence,when the content of Si obtained by ICP-AES is subtracted by the Siamount forming the silicon particles, the Si amount in the silicatephase can be quantitatively obtained. In addition, as the standardsubstance necessary for the quantitative determination, a mixturecontaining a silicate phase and silicon particles, the Si contents ofwhich are each known in advance, at a predetermined rate may be used.

Hereinafter, desired measurement conditions of Si-NMR are shown.

<Si-NMR Measurement Conditions>

Measurement apparatus: manufactured by Varian, Solid-state nuclearmagnetic resonance spectroscopy apparatus (INOVA-400)

Probe: Varian 7 mm CPMAS-2

MAS: 4.2 kHz

MAS Rate: 4 KHz

Pulse: DD (45° pulse+signal acquisition time 1H decoupling)

Repetition time: 1,200 to 3,000 seconds

Observation width: 100 KHz

Center of observation: approximately −100 ppm

Signal acquisition time: 0.05 seconds

Number of scans: 560

Sample amount: 207.6 mg

The silicate composite particles can be recovered from a battery by thefollowing method. First, after the battery is disassembled, a negativeelectrode is taken out and washed with anhydrous ethyl methyl carbonateor dimethyl carbonate, so that an electrolyte liquid is removed.Subsequently, a negative electrode mixture is peeled away from copperfoil and then pulverized with a mortar, so that a sample powder isobtained. Next, after being dried for one hour in a dry atmosphere, thesample powder is immersed in a weakly boiled 6M hydrochloric acid for 10minutes, so that alkali metals, such as Na and Li, contained in abinding agent or the like are removed. Subsequently, after being washedwith ion-exchanged water, the sample powder is processed by filtrationand is then dried at 200° C. for one hour. Next, by firing performed at900° C. in an oxygen atmosphere to remove carbon components, thesilicate composite particles can only be isolated.

A secondary battery according to an embodiment of the present disclosureincludes a positive electrode, a negative electrode, an electrolyte, anda separator provided between the positive electrode and the negativeelectrode, and the negative electrode includes a collector and anegative electrode active material layer. In this embodiment, thenegative electrode active material layer contains the negative electrodeactive material (or the silicate composite particles) for a secondarybattery according to the embodiment of the present disclosure. Thenegative electrode active material layer may also contain another activematerial (such as a lithium titanium oxide, a silicone active material,or a carbon material including a graphite).

In a cross-sectional SEM-EDX analysis (scanning electronmicroscope-energy dispersive X-ray analysis) of the negative electrodeactive material layer in a discharge state, a ratio: I(Na/O) of anintensity which belongs to Na to an intensity which belongs to O is, forexample, 0.02 to 0.7, and a ratio: I(K/O) of an intensity which belongsto K to the intensity which belongs to O is, for example, 0.01 to 0.4.In addition, a ratio: I(Na/K) of the intensity which belongs to Na tothe intensity which belongs to K is, for example, 0.1 to 18. In the casedescribed above, in general, an atomic ratio: Na/K of Na to K containedin the silicate phase is regarded to be in a range of 0.1 to 7. Inaddition, with respect to the total amount of the elements other than Ocontained in the silicate phase, the content of Na and the content of Kare each regarded to be 7 percent by mole or more. In addition, theintensity which belongs to a predetermined element indicates a netintensity excluding a background, and hereinafter, the intensity will bedescribed in the same manner as described above.

In the cross-sectional SEM-EDX analysis of the negative electrode activematerial layer in a discharge state, a ratio: I(Ca/O) of an intensitywhich belongs to Ca to the intensity which belongs to O is, for example,0.5 or less. In this case, the content of Ca with respect to the totalamount of the elements other than O contained in the silicate phase isregarded to be 15 percent by mole or less.

In addition, the discharge state indicates a state in which when a ratedcapacity of the battery is C, discharge is performed to a charge state(SOC: State of Charge) of 0.05×C or less. For example, the dischargestate indicates a state in which discharge is performed at a constantcurrent of 0.05 C to a lower limit voltage.

Hereinafter, desired measurement conditions of the cross-sectionalSEM-EDX analysis are shown.

<SEM-EDX Measurement Conditions>

Processing apparatus: manufactured by JEOL, SM-09010 (Cross SectionPolisher)

Processing condition: accelerating voltage 6 kV

Current: 140 μa

Degree of vacuum: 1×10⁻³ to 2×10⁻³ Pa

Measurement apparatus: electron microscope, manufactured by Hitachi,Ltd., SU-70

Accelerating voltage in analysis: 10 kV

Field: free mode

Probe current mode: Medium

Probe current range: High

Anode Ap.: 3

OBJ Ap.: 2

Analysis area: 1 μm square

Analysis software: EDAX Genesis

CPS: 20500

Lsec: 50

Time constant: 3.2

In addition, the quantitative determination of each element of thesilicate composite particle contained in the negative electrode activematerial layer in a discharge state may also be performed by, besidesthe SEM-EDX analysis, an Auger electron spectroscopy (AES), a laserabrasion ICP mass analysis (LA-ICP-MS), an X-ray photoelectronspectroscopy (XPS), or the like.

Since the silicate phase has a poor electron conductivity, the electricconductivity of the silicate composite particle also tends to be low. Onthe other hand, when an electrically conductive layer is formed using anelectrically conductive material to cover the surface of the silicatecomposite particle, the electric conductivity of the silicate compositeparticle can be remarkably increased. As the electrically conductivematerial, an electrically conductive carbon material is preferable. Theelectrically conductive carbon material preferably contains at least oneselected from the group consisting of a carbon compound and acarbonaceous material.

The electrically conductive layer formed using the electricallyconductive material is preferably thin so as not to actually influencethe average particle diameter of the silicate composite particles. Inorder to ensure the electric conductivity and in consideration of thediffusivity of lithium ions, the thickness of the electricallyconductive layer is preferably 1 to 200 nm and more preferably 5 to 100nm. The thickness of the electrically conductive layer may be measuredby a cross-sectional observation of particles using a SEM or a TEM.

As the carbon compound, for example, there may be mentioned a compoundcontaining carbon and hydrogen or a compound containing carbon,hydrogen, and oxygen. As the carbonaceous material, for example, anamorphous carbon having a low crystallinity or a graphite having a highcrystallinity may be used. As the amorphous carbon, for example, acarbon black, a coal, a coke, a charcoal, or an active carbon may beused. As the graphite, for example, a natural graphite, an artificialgraphite, or graphitized mesophase carbon particles may be mentioned.Among those mentioned above, since having a low hardness and a highbuffer function for the silicon particles, the volume of each of whichis changed in charge/discharge, an amorphous carbon is preferable. Asthe amorphous carbon, a graphitizable carbon (soft carbon) may be used,or a non-graphitizable carbon (hard carbon) may also be used. As thecarbon black, for example, acetylene black or Ketjen black may bementioned.

Next, a method for manufacturing silicate composite particles will bedescribed in detail.

Process (i)

As a raw material of a silicate, a raw material mixture containing a Siraw material, a Na raw material, and a K raw material at a predeterminedrate may be used. The raw material mixture may also contain, if needed,a Li raw material, a raw material of the group 2 element, and/or a rawmaterial of the element M. The raw material mixture is melted, and itsmolten liquid is allowed to pass through metal rollers to form flakes,so that the silicate is formed. In addition, for example, if theproductivity of a silicon pulverization step is improved, the silicatein the form of flakes may also be used after being crystallized toincrease the hardness. Alternatively, without melting the raw materialmixture, the raw material mixture may be fired at a temperature of themelting point or less so as to manufacture the silicate by a solid-statereaction.

As the Si raw material, a silicon oxide (such as SiO₂) may be used. Asthe raw materials of the alkali metals, the group 2 element, and theelement M, for example, carbonate salts, oxides, hydroxides, hydrides,nitrate salts, or sulfate salts thereof may be used. Among thosementioned above, for example, the carbonate salts, the oxides, and thehydroxides are preferable.

Process (ii)

Next, a silicon raw material is blended with the silicate to form acomposite. For example, through the following steps (a) to (c), thesilicate composite particles are formed.

Step (a)

First, a powder of the silicon raw material and a powder of the silicateare mixed, for example, at a mass ratio of 20:80 to 95:5. As the siliconraw material, course silicon particles having an average particlediameter of several to several tens of micrometers may be used.

Step (b)

Next, by using a pulverizing machine, such as a ball mill, the mixtureof the silicon raw material and the silicate is pulverized into fineparticles while being stirred. Even when the silicate is crystallized,by this pulverization step, the silicate is changed into an amorphousstate. In this step, by adding an organic solvent to the mixture, a wetpulverization is preferably performed. A predetermined amount of theorganic solvent may be charged at a time at an initial pulverizationstage, or the organic solvent may be charged several times in anintermittent manner in the pulverization step. The organic solventfunctions to prevent an object to be pulverized from adhering to aninner wall of a pulverization container.

As the organic solvent, for example, an alcohol, an ether, a fatty acid,an alkane, a cycloalkane, a silicate ester, or a metal alkoxide may beused.

After being separately pulverized into fine particles, the silicon rawmaterial and the silicate may be mixed with each other. In addition,after silicon nanoparticles and silicate nanoparticles are formedwithout using a pulverization machine, those nanoparticles may be mixedwith each other. For the formation of nanoparticles, a known method,such as a vapor phase method (such as a plasma method) or a liquid phasemethod (such as a liquid-phase reduction method) may be used.

Step (c)

Next, for example, in an inert atmosphere (such as an atmosphere ofargon or nitrogen), while being pressurized, the mixture in the form offine particles is sintered by heating at 450° C. to 1,000° C. For thesintering, a hot press device or a sintering apparatus which performsdischarge plasma sintering while a pressure is applied in an inertatmosphere may be used. During the sintering, the silicate is melted toflow so as to fill the voids formed between the silicon particles. As aresult, a dense block sintered body in which the silicate phasefunctions as a sea portion and the silicon particles function as islandportions can be obtained.

By pulverizing the sintered body thus obtained, the silicate compositeparticles are obtained. By appropriately selecting the pulverizationconditions, silicate composite particles having a predetermined averageparticle diameter can be obtained. The average particle diameter of thesilicate composite particles is, for example, 1 to 20 μm. The averageparticle diameter of the silicate composite particles indicates aparticle diameter (volume average particle diameter) at a volumeintegrated value of 50% in a volume-basis particle size distributionmeasured by a laser diffraction scattering method.

Process (iii)

Next, the surfaces of the silicate composite particles may be at leastpartially covered with an electrically conductive material to formelectrically conductive layers. The electrically conductive material ispreferably electrochemically stable, and an electrically conductivecarbon material is preferable. As a method to cover the surfaces of thesilicate composite particles by an electrically conductive carbonmaterial, for example, there may be mentioned a CVD method using ahydrocarbon gas, such as acetylene or methane, as a raw material or amethod in which after the silicate composite particles are mixed with acoal pitch, a petroleum pitch, a phenol resin, or the like, heating isperformed for carbonization. In addition, a carbon black may be adheredto the surfaces of the silicate composite particles.

Process (iv)

A step of washing the composite particles (the case in which theelectrically conductive layers are provided on the surfaces thereof isincluded) may also be performed with an acid. For example, when thecomposite particles are washed with an acidic aqueous solution, a smallamount of alkaline components which may be present on the surfaces ofthe composite particles can be dissolved and removed. As the acidicaqueous solution, for example, an aqueous solution of an inorganic acid,such as hydrochloric acid, hydrofluoric acid, sulfuric acid, nitricacid, phosphoric acid, or carbonic acid, or an aqueous solution of anorganic acid, such as citric acid or acetic acid may be used. However,when the silicate phase contains a group 2 element, the washing of thecomposite particles with an acidic aqueous solution is not so muchnecessary.

FIG. 1 is a schematic cross-sectional view showing one example of asilicate composite particle 20. The silicate composite particle 20generally includes a mother particle 23 which is a secondary particleformed by aggregation of a plurality of primary particles 24. Theprimary particles 24 each include a silicate phase 21 and siliconparticles 22 dispersed in the silicate phase 21. The mother particle 23has sea-island structures each of which includes the silicate phase 21and the fine silicon particles 22 dispersed in the matrix thereof. Thesilicon particles 22 are approximately uniformly dispersed in thesilicate phase 21. In the silicate composite particle, voids are hardlyobserved, and the void rate is at most less than 5 percent by volume orless.

In the mother particle 23, at at least a part of the interface betweenadjacent primary particles 24, a carbon region 25 can exist. The carbonregion 25 contains, as a primary component, a residue of the organicsolvent used in the manufacturing step (b) of the silicate compositeparticles. In addition, in the formation of the silicate compositeparticles, an organic solvent may also be used in order to form thecarbon regions. The carbon region 25 suppresses a decrease in ionconductivity of the silicate composite particle and, in addition,contributes to release of a stress generated in the silicate phase inassociation with the expansion and the contraction of the siliconparticles in charge/discharge.

The silicate composite particle 20 further includes an electricallyconductive material (electrically conductive layer 26) which covers atleast a part of the surface of the mother particle 23. In this case, anend portion of the carbon region 25 at a surface side of the motherparticle 23 is preferably in contact with the electrically conductivelayer 26. Accordingly, a preferable electrically conductive network isformed from the surface to the inside of the mother particle 23.

The silicate phase 21 and the silicon particle 22 are each formed byaggregation of fine particles. The silicate phase 21 may be formed fromparticles finer than those for the silicon particle 22. In this case, inan X-ray diffraction (XRD) pattern of the silicate composite particle20, the ratio of the integrated value of the diffraction peak of each ofall the other elements to the integrated value of the diffraction peakwhich belongs to the (111) plane of single Si is, for example, 0.5 orless.

In order to increase the capacity and to improve the cyclecharacteristics, the content of the silicon particles 22 (single Si)occupied in the mother particle 23 measured by Si-NMR is preferably 20to 95 percent by mass and more preferably 35 to 75 percent by mass.Accordingly, a high charge/discharge capacity can be ensured.

The mother particle 23 may contain other components besides the silicatephases 21, the silicon particles 22, and the carbon regions 25. Forexample, the silicate composite particle 20 may contain a small amountof crystalline or amorphous SiO₂. However, the content of SiO₂ occupiedin the mother particle 23 measured by Si-NMR is preferably, for example,less than 7 percent by mass. In addition, in order to improve thestrength of the mother particle 23, a reinforcing material, such as anoxide including ZrO₂ or a carbide functioning as the carbon region 25,may be contained at less than 10 percent by weight with respect to themother particle 23.

Since containing Na and K, the silicate phase 21 has a composition whichis not likely to cause phase splitting. When the phase splitting onceoccurs, crystallization is liable to occur, and when crystals aregenerated, the fluidity of the silicate phase is degraded. Hence, nophase splitting preferably occurs.

The average particle diameter of the primary particles 24 is, forexample, 0.2 to 10 μm and is preferably 2 to 8 μm. Accordingly, thestress caused by the volume change of the silicate composite particlesin association with charge/discharge is more likely to be released, andpreferable cycle characteristics are likely to be obtained. In addition,since the surface area of the silicate composite particle becomesappropriate, a decrease in capacity by a sub-reaction with theelectrolyte is also suppressed.

The average particle diameter of the primary particles 24 can bemeasured by observing the cross-section of the silicate compositeparticle using a SEM. In particular, the average particle diameter canbe obtained by averaging the diameters of cross-sectional equivalentcircles (circles each having the same area as the cross-sectional areaof the primary particle) of arbitrary 100 primary particles 24.

The average particle diameter of the silicon particles 22 is, before aninitial charge, 500 nm or less, preferably 200 nm or less, and morepreferably 50 nm or less. When the silicon particles 22 areappropriately minaturized, the volume change in charge/discharge isdecreased, and the structural stability is improved. The averageparticle diameter of the silicon particles 22 can be measured byobserving the cross-sections of the silicate composite particles using aSEM or a TEM. In particular, the average particle diameter can beobtained by averaging the maximum diameters of arbitrary 100 siliconparticles 22.

Hereinafter, the negative electrode, the positive electrode, theelectrolyte, and the separator of the secondary battery according to theembodiment of the present disclosure will be described.

[Negative Electrode]

The negative electrode includes, for example, a negative electrodecollector and a negative electrode active material layer. The negativeelectrode active material layer contains a negative electrode activematerial. The negative electrode active material at least containssilicate composite particles. The negative electrode active materiallayer is formed on at least one surface of the negative electrodecollector. The negative electrode active material layer may be formed onone surface of the negative electrode collector or may also be formed oneach of two surfaces thereof. The negative electrode active materiallayer is formed such that a negative electrode slurry in which anegative electrode mixture is dispersed in a dispersion medium isapplied on the surface of the negative electrode collector and thendried. A coating film thus dried may be rolled, if needed. The negativeelectrode active material layer may further contain, as an arbitrarycomponent, a binding agent, an electrically conductive agent, athickening agent, and/or the like.

The negative electrode active material may further contain anotheractive material. As the another active material, for example, acarbonaceous active material which electrochemically occludes andreleases lithium ions is preferably contained. Since the volume of thesilicate composite particles is expanded and contracted in associationwith charge/discharge, when the rate of the silicate composite particlesoccupied in the negative electrode active material is increased, acontact failure between the negative electrode active material and thenegative electrode collector is liable to occur in association withcharge/discharge. On the other hand, since the silicate compositeparticles are used together with the carbonaceous active material, whilea high capacity of the silicon particles is imparted to the negativeelectrode, excellent cycle characteristics can be achieved. The rate ofthe silicate composite particles to the total of the silicate compositeparticles and the carbonaceous active material is preferably, forexample, 3 to 30 percent by mass. Accordingly, the increase in capacityand the improvement in cycle characteristics are likely to besimultaneously obtained.

As the carbonaceous active material, for example, a graphite, agraphitizable carbon (soft carbon), or a non-graphitizable carbon (hardcarbon) may be mentioned. Among those mentioned above, a graphite whichis excellent in charge/discharge stability and which has a lowirreversible capacity is preferable. The graphite indicates a materialhaving a graphite type crystalline structure and may include a naturalgraphite, an artificial graphite, graphitized mesophase carbonparticles, and the like. The carbonaceous active material may be usedalone, or at least two types thereof may be used in combination.

As the negative electrode collector, for example, a porelesselectrically conductive substrate (such as metal foil) or a porouselectrically conductive substrate (such as a mesh, a net, or a punchingsheet) may be used. As a material of the negative electrode collector,for example, there may be mentioned stainless steel, nickel, a nickelalloy, copper, or a copper alloy. Although not particularly limited, inconsideration of the balance between the strength of the negativeelectrode and the reduction in weight thereof, the thickness of thenegative electrode collector is preferably 1 to 50 μm and morepreferably 5 to 20 μm.

The binding agent preferably contains, as at least a first resin, atleast one selected from the group consisting of a polyacrylic acid, apolyacrylate salt, and derivatives thereof. As the polyacrylate salt, aLi salt or a Na salt is preferably used. In particular, a cross-linkablepoly(lithium acrylate) is preferably used.

The content of the first resin in the negative electrode active materiallayer is preferably 2 percent by mass or less and more preferably 0.2 to2 percent by mass.

A second resin may be used in combination with the first resin. As thesecond resin, for example, there may be mentioned a fluorine resin, suchas a polytetrafluoroethylene or a poly(vinylidene fluoride) (PVDF); apolyolefin, such as a polyethylene or a polypropylene; a polyamideresin, such as an aramid resin; a polyimide resin, such as a polyimideor a poly(amide imide); a vinyl resin such as a poly(vinyl acetate); apoly(vinyl pyrrolidone); a poly(ether sulfone); or a rubber material,such as a styrene-butadiene copolymer (SBR). Those mentioned above maybe uses alone, or at least two types thereof may be used in combination.The second resin may be an acrylic resin other than the first resin. Asthe acrylic resin other than the first resin, for example, there may bementioned a poly(methyl acrylate), an ethylene-acrylic acid copolymer, apolyacrylonitrile, a poly(methacrylic acid), a poly(methacrylate salt),or a derivative thereof.

As the electrically conductive agent, for example, there may bementioned a carbon black such as an acetylene black; electricallyconductive fibers, such as carbon fibers or metal fibers; a carbonfluoride; a metal powder of aluminum or the like; electricallyconductive whiskers of zinc oxide or potassium titanate; an electricallyconductive metal oxide such as titanium oxide; or an organicelectrically conductive material such as a phenylene derivative. Thosementioned above may be used alone, or at least two types thereof may beused in combination.

As the thickening agent, for example, there may be mentioned a cellulosederivative (including a cellulose ether), such as a carboxymethylcellulose (CMC), a modified material thereof (including a salt, such asa Na salt), or a methylcellulose; a saponified product of a polymer,such as a poly(vinyl alcohol), having a vinyl acetate unit; or apolyether (such as a poly(alkylene oxide) including a poly(ethyleneoxide)). Those mentioned above may be used alone, or at least two typesthereof may be used in combination.

Although the dispersion medium is not particularly limited, for example,there may be mentioned water; an alcohol such as ethanol; an ether suchas tetrahydrofuran; an amide such as dimethylformamide;N-methyl-2-pyrrolidone (NMP), or a mixed solvent thereof.

[Positive Electrode]

The positive electrode includes, for example, a positive electrodecollector and a positive electrode active material layer formed on atleast one surface of the positive electrode collector. The positiveelectrode active material layer is formed such that a positive electrodeslurry in which a positive electrode mixture is dispersed in adispersion medium is applied on the surface of the positive electrodecollector and then dried. A coating film thus dried may be rolled, ifneeded. The positive electrode active material layer may be formed onone surface of the positive electrode collector or may also be formed oneach of two surfaces thereof.

The positive electrode mixture contains, as an essential component, apositive electrode active material, and, as an arbitrary component, mayalso contain a binding agent, an electrically conductive agent, and/orthe like.

As the positive electrode active material, a lithium composite metaloxide may be used. As the lithium composite metal oxide, for example,Li_(a)CoO₂, Li_(a)NiO₂, Li_(a)MnO₂, Li_(a)Co_(b)Ni_(1-b)O₂,Li_(a)Co_(b)M_(1-b)O_(c), Li_(a)Ni_(1-b)M_(b)O_(c), Li_(a)Mn₂O₄,Li_(a)Mn_(2-b)M_(b)O₄, LiMePO₄, or Li₂MePO₄F may be mentioned. In thecase described above, M is at least one selected from the groupconsisting of Na, Mg, Sc, Y, Mn, Fe, Co, Ni, Cu, Zn, Al, Cr, Pb, Sb, andB. Me includes at least one transition metal (for example, including atleast one selected from the group consisting of Mn, Fe, Co, and Ni).0≤a≤1.2, 0≤≤b≤0.9, and 2.0≤c≤2.3 hold. In addition, the a value whichindicates a molar ratio of lithium is a value in a discharge state andis increased and decreased by charge/discharge in accordance with avalue immediately after the formation of the active material.

As the binding agent and the electrically conductive agent, materialssimilar to those described by way of example for the negative electrodemay also be used. As the electrically conductive agent, a graphite, suchas a natural graphite or an artificial graphite, may be used.

The shape and the thickness of the positive electrode collector may beselected from the shape and the thickness range, respectively, inaccordance with the negative electrode collector. As a material of thepositive electrode collector, for example, stainless steel, aluminum, analuminum alloy, or titanium may be mentioned.

[Electrolyte]

The electrolyte contains a solvent and a lithium salt dissolved in thesolvent. The concentration of the lithium salt in the electrolyte is,for example, 0.5 to 2 mol/L. The electrolyte may also contain a knownadditive.

As the solvent, an aqueous solvent or a nonaqueous solvent may be used.As the nonaqueous solvent, for example, a cyclic carbonate ester, achain carbonate ester, or a cyclic carboxylate ester may be used. As thecyclic carbonate ester, for example, propylene carbonate (PC) orethylene carbonate (EC) may be mentioned. As the chain carbonate ester,for example, diethyl carbonate (DEC), ethyl methyl carbonate (EMC), ordimethyl carbonate (DMC) may be mentioned. As the cyclic carboxylateester, for example, γ-butyrolactone (GBL) or γ-valerolactone (GVL) maybe mentioned. The nonaqueous solvent may be used alone, or at least twotypes thereof may be used in combination.

As the lithium salt, for example, a lithium salt (such as LiClO₄,LiAlCl₄, or LiB₁₀Cl₁₀) of an acid containing chlorine, a lithium salt(such as LiPF₆, LiBF₄, LiSbF₆, LiAsF₆, LiCF₃SO₃, or LiCF₃CO₂) of an acidcontaining fluorine, a lithium salt (such as (LiN(SO₂F)₂, LiN(CF₃SO₂)₂,LiN(CF₃SO₂) (C₄F₉SO₂), or LiN(C₂F₅SO₂)₂) of an imide containingfluorine, or a lithium halide (such as LiCl, LiBr, or LiI) may be used.The lithium salt may be used alone, or at least two types thereof may beused in combination.

[Separator]

In general, the separator is provided between the positive electrode andthe negative electrode. The separator has a high ion permeability, anappropriate mechanical strength, and an appropriate insulating property.As the separator, a fine porous thin film, a woven cloth, or a non-wovencloth may be used. As a material of the separator, for example, apolyolefin, such as a polypropylene or a polyethylene, may be used.

As one example of the structure of the secondary battery, a structure inwhich an electrolyte and an electrode group formed by winding a positiveelectrode, a negative electrode, and at least one separator are receivedin an exterior package may be mentioned. Instead of the winding typeelectrode group, a laminate type electrode group in which positiveelectrodes and negative electrodes are laminated with separatorsinterposed therebetween may also be used. In addition, an electrodegroup having another structure may also be used. The secondary batterymay have any shape, such as a cylindrical type, a square type, a cointype, a button type, or a laminate type.

FIG. 2 is a partially notched schematic perspective view of a squaresecondary battery according to one embodiment of the present disclosure.The battery includes a bottom-closed square battery case 4, an electrodegroup 1 and an electrolyte (not shown) received in the battery case 4,and a sealing plate 5 which seals an opening of the battery case 4. Theelectrode group 1 includes a long belt-shaped negative electrode, a longbelt-shaped positive electrode, and separators interposed therebetween.The negative electrode, the positive electrode, and the separators arewound around a flat winding core, and when the winding core is removed,the electrode group 1 is formed. The sealing plate 5 has a liquid chargeport sealed with a sealing plug 8 and a negative electrode terminal 6insulated from the sealing plate 5 by a gasket 7.

To a negative electrode collector of the negative electrode, one end ofa negative electrode lead 3 is fitted by welding or the like. To apositive electrode collector of the positive electrode, one end of apositive electrode lead 2 is fitted by welding or the like. The otherend of the negative electrode lead 3 is electrically connected to thenegative electrode terminal 6. The other end of the positive electrodelead 2 is electrically connected to the sealing plate 5. At an upperportion of the electrode group 1, a resin-made frame body is disposed soas to separate not only between the electrode group 1 and the sealingplate 5 but also between the negative electrode lead 3 and the batterycase 4.

Hereinafter, although the present invention will be described withreference to examples and comparative examples, the present invention isnot limited to the following examples.

Example 1

[Preparation of Silicate Composite Particles]

Process (i)

Sodium carbonate, potassium carbonate, and silicon dioxide were mixed tohave a molar ratio Na/K/Si of 50/7.1/42.9, so that a raw materialmixture was obtained. The raw material mixture was melted by heating at1,500° C. for 5 hours in an inert atmosphere, and a molten liquid wasallowed to pass through metal rollers to form flakes, so that a silicatecontaining Na, K, and Si was obtained. The silicate thus obtained waspulverized to have an average particle diameter of 10 μm.

Process (ii)

The silicate having an average particle diameter of 10 μm and a siliconraw material (3N, average particle diameter: 10 μm) were mixed togetherto have a mass ratio of 50:50. After the mixture was charged in a pot(formed from SUS, volume: 500 mL) of a planetary ball mill (manufacturedby Fritsch, P-5), and 24 SUS-made balls (diameter: 20 mm) were alsocharged in the pot, the pot was lidded, and a pulverization treatmentwas performed on the mixture at 200 rpm for 25 hours. In addition, tothe mixture charged in the pot, ethanol was added as an organic solvent.The addition amount of the ethanol was set to 0.016 parts by mass withrespect to 100 parts by mass of the mixture of the silicate and thesilicon raw material.

Subsequently, after the mixture of the silicate and the silicon rawmaterial was recovered, firing was performed at 600° C. for 4 hours inan inert atmosphere while the mixture was pressurized, so that asintered body of the silicate was obtained.

Process (iii)

Subsequently, after the sintered body was pulverized into silicatecomposite particles and was allowed to pass through a 40-μm mesh, a coalpitch (manufactured by JFE Chemicals Corporation, MCP250) was mixedtherewith, and a mixture of the silicate composite particles and thepitch was fired at 800° C. for 5 hours in an inert atmosphere to coverthe surfaces of the composite particles with an electrically conductivecarbon, so that electrically conductive layers were formed. The coveringamount of the electrically conductive layer was set to 5 percent by masswith respect to the total mass of the composite particle and theelectrically conductive layer. Next, by using a sieve, particles(silicate composite particles A1) which had an average particle diameterof 10 μm and which included the silicate composite particles and theelectrically conductive layers formed on the surfaces thereof wereclassified. The thickness of the electrically conductive layer wasestimated to approximately 200 nm.

Examples 2 to 15

Except for that a raw material mixture was obtained by mixing materialstogether so that a molar ratio of each element had the rate shown inTable 1, silicate composite particles A2 to A15 were formed in a mannersimilar to that of Example 1.

Besides the sodium carbonate, the potassium carbonate, and the silicondioxide, lithium carbonate, calcium carbonate, barium carbonate, zincoxide, boron oxide, aluminum oxide, and phosphoric acid were also used.

Comparative Examples 1 to 4

Except for that a raw material mixture was obtained by mixing materialstogether so that a molar ratio of each element had the rate shown inTable 1, silicate composite particles B1 to B4 were formed in a mannersimilar to that of Example 1.

In Table 1, the silicate composite particles obtained in Examples 1 to15 were called A1 to A15, respectively, and the silicate compositeparticles obtained in Comparative Examples 1 to 4 were called B1 to B4,respectively.

TABLE 1 Crystal- Cycle Na K Li Ca Ba Zn B Al P Si Na/K Viscos- lizationVoid charac- Percent Percent Percent Percent Percent Percent PercentPercent Percent Percent Atomic ity rate rate teristics Sample by mole bymole by mole by mole by mole by mole by mole by mole by mole by moleratio Poise % % Index B1 47.8 0 0 2.6 0.7 1.5 1.5 1.5 0 44.4 — 8.3 3010.0 100 B2 57.1 0 0 0 0 0 0 0 0 42.9 — 8.0 30 11.0  91 B3 0 57.1 0 0 00 0 0 0 42.9 0 8.5 30 10.0  95 B4 0 0 47.8 2.6 0.7 1.5 1.5 1.5 0 44.4 —8.0 80 20.0  80 A1 50 7.1 0 0 0 0 0 0 0 42.9 7.04 7.2 20  4.0 115 A2 7.150.0 0 0 0 0 0 0 0 42.9 0.14 7.0 20  3.0 118 A3 25.0 25.0 7.1 0 0 0 0 00 42.9 1.00 7.0 20  1.0 128 A4 21.4 21.4 14.3 1.4 0 0 0 0 0 41.4 1.006.0 20  1.0 131 AS 21.4 21.4 14.3 7.1 0 0 0 0 0 35.7 1.00 6.8 10  1.0120 A6 21.4 21.4 14.3 1.4 0 7.1 0 0 0 34.3 1.00 7.0 20  0.9 130 A7 20.020.0 13.3 1.3 0 0 0 0 13.3 32.0 1.00 6.8 20  1.2 130 A8 16.4 16.4 14.92.6 0.7 1.5 1.5 1.5 0 44.4 1.00 6.7  0  1.0 140 A9 23.9 23.9 0 7.5 0.71.5 1.5 1.5 0 39.6 1.00 6.8  0  2.0 130 A10 40.3 7.5 0 2.6 0.7 1.5 1.51.5 0 44.4 5.37 7.3 20  4.0 113 A11 14.9 32.8 0 7.5 0.7 1.5 1.5 1.5 039.6 0.45 7.0  0  2.0 135 A12 7.5 40.3 0 2.6 0.7 1.5 1.5 1.5 0 44.4 0.197.0 20  1.5 136 A13 27.1 4.5 18 5.3 0 0 0 0 0 45.1 6.00 7.0 10  3.0 116A14 16.5 16.5 16.5 5.3 0 0 0 0 0 45.1 1.00 7.0 10  4.0 117 A15 20.2 7.60.84 2.9 0.8 1.7 1.7 1.7 0 62.6 2.66 7.0 20  2.0 125

[Formation of Negative Electrode]

The silicate composite particles provided with the electricallyconductive layers and a graphite were mixed at a mass ratio of 5:95, andthis mixture was used as a negative electrode active material. Afterwater was added to a negative electrode mixture containing the negativeelectrode active material, a sodium carboxymethyl cellulose (CMC-Na), astyrene-butadiene rubber (SBR), and a poly(lithium acrylate) salt at amass ratio of 96.5:1:1.5:1, stirring was performed using a mixer(manufactured by PRIMIX Corporation, T. K. Hivis Mix) to prepare anegative electrode slurry. Next, after the negative electrode slurry wasapplied on surfaces of copper foil in an amount of 190 g/m², and coatingfilms thus formed were dried, rolling was performed, so that a negativeelectrode in which negative electrode active material layers each havinga density of 1.5 g/cm³ were formed on the two surfaces of the copperfoil was formed.

[Formation of Positive Electrode]

After N-methyl-2-pyrolidone (NMP) was added to a positive electrodemixture containing lithium cobalt oxide, an acetylene black, and apoly(vinylidene fluoride) at a mass ratio of 95:2.5:2.5, stirring wasperformed using a mixer (manufactured by PRIMIX Corporation, T.K. HivisMix) to prepare a positive electrode slurry. Next, after the positiveelectrode slurry was applied on surfaces of aluminum foil, and coatingfilms thus formed were dried, rolling was performed, so that a positiveelectrode in which positive electrode active material layers each havinga density of 3.6 g/cm³ were formed on the two surfaces of the aluminumfoil was formed.

[Preparation of Nonaqueous Electrolyte Liquid]

In a mixed solvent containing ethylene carbonate (EC) and diethylenecarbonate (DEC) at a volume ratio of 3:7, LiPF₆ was dissolved at aconcentration of 1.0 mol/L, so that a nonaqueous electrolyte liquid wasformed.

[Formation of Secondary Battery]

Tabs were fitted to the respective electrodes, and the positiveelectrode and the negative electrode were spirally wound with theseparators interposed therebetween so that the tabs were each located atan outermost circumferential portion, so that an electrode group wasformed. After the electrode group was inserted into an aluminum laminatefilm-made exterior package and then vacuum-dried at 105° C. for 2 hours,the nonaqueous electrolyte liquid was charged, and an opening of theexterior package was sealed, so that a secondary battery was obtained.

In Table 1, batteries obtained using the particles A1 to A15 and theparticles B1 to B4 were also called Al to A15 and B1 to B4,respectively, in a manner similar to that described above.

[Analysis of Silicate Composite Particle]

(A) Cross-Sectional Observation

When a cross-section of the silicate composite particle was observedusing a scanning electron microscope (SEM), it was confirmed that thesilicate composite particle was composed of a secondary particle formedfrom aggregated primary particles (average particle diameter: 3 μm). Inaddition, it was also confirmed that in a matrix of an oxide phasecontaining Na, K, and Si, silicon particles having an average particlediameter of 50 nm were approximately uniformly dispersed.

(b) Quantitative Determination of Elements Other than Si

The contents of Na, K, B, and Al contained in the silicate phase and thecontent of Ca contained therein were quantified in accordance with JIS83105 (1995) (analysis method of borosilicate glass) and JIS 83101(1995) (analysis method of soda-lime glass), respectively. The contentsof the other elements contained in the silicate phase were eachcalculated by an ICP atomic emission spectrometry (ICP-AES).

(c) Quantitative Determination of Si Element Forming Silicon Particles

A Si amount forming the silicon particles was quantified by Si-NMR.

(d) Quantitative Determination of Si Element Contained in Silicate Phase

From the total Si amount obtained in the above (b) by ICP-AES, the Siamount forming the silicon particles was subtracted, so that a rate ofSi with respect to the total amount of the elements other than Ocontained in the silicate phase was obtained. In addition, the contentof SiO₂ was less than the lower detection limit.

When the contents of the elements contained in the silicate phase of theparticles A1 were analyzed by the above (b) to (d), the content of Na,the content of K, and the content of Si with respect to the total amountof the elements other than O contained in the silicate phase were 49.3percent by mole, 7.35 percent by mole, and 42.9 percent by mole,respectively. In all the examples, the analysis value of each elementwas not so much different from a charge rate show in Table 1.

(e) Crystallization Rate

The silicate (hereinafter, also referred to as “glass flakes” in somecases) in the form of flakes obtained in Process (i) was heated at 930°C. for 10 hours, so that a standard sample in which a crystallizationrate of the silicate phase was 100% was obtained. After the standardsample was pulverized, by an X-ray diffraction method (XRD), a totalvalue (A) of integrated values of all diffraction peaks in a 20 range of10° to 40° was obtained. In addition, after the glass flakes were heatedat 600° C. for 5 hours and then pulverized, a total value (B) ofintegrated values of all diffraction peaks in a 20 range of 10° to 40°was obtained by XRD. A rate of B with respect to A which was calculatedby (B/A)×100 was regarded as the crystallization rate (%).

(f) Viscosity

The glass flakes were melted at 1,400° C. and then dripped on a metalsurface for rapid cooling, so that glass cullet was formed. After theglass cullet was polished, cutting was performed, so that test pieceshaving a thickness of 5 to 8 mm and a diameter of 20 to 30 mm wereobtained. Subsequently, the test piece was set in a wide-rangeviscometer manufactured by OPT Corp., and while the temperature wasincreased at a rate of 5° C./min, a viscosity at 600° C. was calculatedfrom a penetration depth of a ZrO₂ ball, a displacement between parallelplates of the viscometer after the ball penetration, and a rotation rateof the parallel plates after the displacement thereof.

[Initial Charge/Discharge]

After a constant current charge was performed on each battery at 25° C.and at a current of 1 It (800 mA) to a voltage of 4.2 V, a constantvoltage charge was performed at a voltage of 4.2 V to a current of 1/20It (40 mA). After a rest period for 10 minutes, a constant currentdischarge was performed at a current of 1 It (800 mA) to a voltage of2.75 V.

After the initial discharge was performed, the battery was disassembled,the negative electrode was taken out, and by using a cross-sectionpolisher (CP), a cross-section of the negative electrode active materiallayer was obtained. From a cross-sectional image of the negativeelectrode active material layer by a SEM, 10 silicate compositeparticles each having a maximum diameter of 5 μm or more were randomlyselected, and an image analysis was performed on each particle, so thatan area rate (void rate) of the voids occupied in the cross-section ofthe composite particle was obtained. In this case, the measured valuesof the 10 composite particles were averaged. In addition, the imageanalysis was performed in a region of the composite particle from apoint apart from the circumferential edge of the cross-section by 1 μmto the inside.

Next, a SEM-EDX analysis of the negative electrode active material layerwas performed on A15. A ratio: I(Na/O) of an intensity which belonged toNa to an intensity which belonged to 0 was 0.3. In addition, a ratio:I(K/O) of an intensity which belonged to K to the intensity whichbelonged to 0 was 0.06. In addition, a ratio: I(Na/K) of the intensitywhich belonged to Na to the intensity which belonged to K was 4.58. Inthe SEM-EDX analysis, beams having an area of 1 μm square were appliedto a particle central portion of the silicate composite particle havinga particle diameter of approximately 10 μm.

[Charge/Discharge Cycle Test]

Charge and discharge were repeatedly performed under the followingconditions.

<Charge>

A constant current charge was performed at 25° C. and at a current of 1It (800 mA) to a voltage of 4.2 V, and subsequently, a constant voltagecharge was performed at a voltage of 4.2 V to a current of 1/20 It (40mA).

<Discharge>

A constant current discharge was performed at 25° C. and at a current of1 It (800 mA) to a voltage of 2.75 V.

A rest period between the charge and the discharge was set to 10minutes. A rate of a discharge capacity at a 100th cycle to a dischargecapacity at a first cycle was regarded as a cycle maintenance rate, thenumber of cycles of Comparative Example 1 was set to 100, and the otherswere normalized. When the cycle maintenance rate is 100 or more, thecycle characteristics were evaluated as good. The evaluation results areshown in Table 1.

From Table 1, it is understood that since Na and K are used incombination, the crystallization of the silicate phase is suppressed,and the viscosity of the silicate in a molten state is decreased. As aresult, the void rate of the silicate composite particle issignificantly reduced, and hence, the cycle maintenance rate isremarkably improved.

Since Na, K, and Li are used in combination, the void rate is furtherreduced, and the cycle maintenance rate tends to be increased.Furthermore, since appropriate amounts of the group 2 element and theelement M are contained, the cycle maintenance rate also tends to beincreased.

INDUSTRIAL APPLICABILITY

According to the present disclosure, a secondary battery having a highcapacity and preferable cycle characteristics can be provided. Thesecondary battery according to the present disclosure is useful as amain power source of a portable communication device, a mobileelectronic device, and the like.

REFERENCE SIGNS LIST

-   -   1 electrode group    -   2 positive electrode lead    -   3 negative electrode lead    -   4 battery case    -   5 sealing plate    -   6 negative electrode terminal    -   7 gasket    -   8 sealing plug    -   20 LSX particle    -   21 silicate phase    -   22 silicon particle    -   23 mother particle    -   24 primary particle    -   25 carbon region    -   26 electrically conductive layer

1. A negative electrode active material for a secondary battery,comprising: silicate composite particles each of which contain asilicate phase and silicon particles dispersed in the silicate phase,wherein the silicate phase is an oxide phase which contains Si, O, andalkali metals, and the alkali metals include at least Na and K.
 2. Thenegative electrode active material according to claim 1, wherein anatomic ratio: Na/K of Na to K contained in the silicate phase is 0.1 to7.
 3. The negative electrode active material according to claim 1,wherein, with respect to a total amount of elements other than Ocontained in the silicate phase, a content of Na is 7 percent by mole ormore, a content of K is 7 percent by mole or more, and a total contentof Na and K is 70 percent by mole or less.
 4. The negative electrodeactive material according to claim 1, wherein the alkali metals furtherinclude Li.
 5. The negative electrode active material according to claim4, wherein a content of the alkali metals with respect to a total amountof elements other than 0 contained in the silicate phase is 80 percentby mole or less, and among the alkali metals, a total content of alkalimetals other than a first alkali metal having a highest content is 7percent by mole or more.
 6. The negative electrode active materialaccording to claim 1, wherein the silicate phase further contains agroup 2 element, and a content of the group 2 element with respect to atotal amount of elements other than 0 contained in the silicate phase is20 percent by mole or less.
 7. The negative electrode active materialaccording to claim 1, wherein the silicate phase further contains anelement M, and the element M is at least one selected from the groupconsisting of B, Al, Zr, Nb, Ta, La, V, Y, Ti, P, Bi, Zn, Sn, Pb, Sb,Co, Er, F, and W.
 8. The negative electrode active material according toclaim 1, wherein, in an X-ray diffraction pattern of the silicatecomposite particles, a ratio of an integrated value of a diffractionpeak of each of all the other elements to an integrated value of adiffraction peak which belongs to the (111) plane of single Si is 0.5 orless.
 9. The negative electrode active material according to claim 1,wherein the silicate composite particles have a void rate of less than 5percent by volume.
 10. A secondary battery comprising: a positiveelectrode; a negative electrode; an electrolyte; and a separatorprovided between the positive electrode and the negative electrode,wherein the negative electrode includes a collector and a negativeelectrode active material layer, and the negative electrode activematerial layer contains the negative electrode active material accordingto claim
 1. 11. The secondary battery according to claim 10, wherein, ina cross-sectional SEM-EDX analysis of the negative electrode activematerial layer in a discharge state, a ratio: I(Na/O) of an intensitywhich belongs to Na to an intensity which belongs to O is 0.02 to 0.7, aratio: I(K/O) of an intensity which belongs to K to the intensity whichbelongs to O is 0.01 to 0.4, and a ratio: I(Na/K) of the intensity whichbelongs to Na to the intensity which belongs to K is 0.1 to
 18. 12. Thesecondary battery according to claim 10, wherein, in a cross-sectionalSEM-EDX analysis of the negative electrode active material layer in adischarge state, a ratio: I(Ca/O) of an intensity which belongs to Ca toan intensity which belongs to O is 0.5 or less.